The present invention relates to organic electroluminescent devices and more particularly to an emission layer with suitable dopants for improving operational stability and efficiency of these devices.
An OLED device includes a substrate, an anode, a hole-transporting layer made of an organic compound, an organic luminescent layer with suitable dopants, an organic electron transport layer, and a cathode. EL devices are attractive because of their low driving voltage, high luminance, wide-angle viewing and capability for full-color flat emission displays. Tang et al. described this multilayer EL device in their commonly assigned U.S. Pat. Nos. 4,769,292 and 4,885,211.
The following patents and publications disclose the preparation of EL devices with improved operational lifetime. Structure modifications, stable cathode and confinement of carriers and their recombination in the emission zone achieved significant improvement in the operation stability of these devices. So et al. discussed an EL device in U.S. Pat. No. 5,853,905 consisting of a single organic emission layer with a mixture of electron transport and hole transport material, sandwiched between anode and cathode. However, this device has low efficiency.
Popovic et al. described an EL device in SPIE Conference proceedings Vol. 3476, p. 68-72, 1998 with improved efficiency and stability by mixing emitting electron transport material and the hole transport material. Chen et al. reported a highly efficient red color device where Alq emission layer is doped with DCJTB. (U.S. Pat. No. 5,908,581 and Micromol. Symp. 25, 490 (1997)).
Hamada et al. reported in Applied Phys. Lett. 75,1682 (1999) a red EL device by doping rubrene and DCM2 dopants into Alq emission layer. With the prospect of using these EL devices in display screens for laptop computers, digital personal organizers, cellular phone etc., there is a need for EL device with much stable operation for luminance and driving voltage.
It is therefore an object of the present invention to provide an EL device with improved operational stability and luminance efficiency.
This object is achieved by an organic luminescent layer for use in an electroluminescent device with improved operating life, comprising:
a) an organic host material capable of sustaining both hole and electron injection and recombination; and
b) at least two dopants:
i) a first dopant capable of accepting energy of electron-hole combinations in the host material; and
ii) a second dopant capable of trapping the holes from the host material; and
c) the first dopant being selected so that the bandgap energy of the first dopant is less than the bandgap energy of the host material; and
d) the second dopant being selected to have a hole trapping energy level above the valence band of the host material.
A feature of the present invention is that by properly selecting the first and second dopants, the operating life of an electroluminescent device incorporating the electroluminescent layer can be significantly improved.
Another feature of the invention is that it can be used to provide an electroluminescent device with improved chromaticity.
Another feature of the invention is that it can be used to provide an electroluminescent device with improved operational stability at high luminance output.
Another feature of the invention is that it can be used with a low voltage drive source.
Quite unexpectedly, it has been found in this invention that synergetic effect of doping exciton trapping dopant and hole trapping dopant was such that better operational stability and efficiency were obtained at lower concentrations of the dopants. This resulted in improvements in operating lifetime and luminance efficiency. With an additional third luminescent dopant for controlling the color of the emission, excellent color chromicity was obtained, which could not be achieved with any other combination.
The devices made in accordance with the present invention are highly stable and have excellent luminance properties.